Actually it is pretty simple.
With few exceptions, any text box in an Action that you can type into is a "text field", which can accept any of the following:
- Plain, literal text
- A KM Variable, in the form of a Variable Token, like "
Variable%YourVariableName%" - Any Token, which can be typed in, or selected from a list by clicking on the "
Tokens" dropdown list - A Calculation, using the "Calculation" token, like this:
%Calculate%1+2%
For example:

The "Insert Token" dropdown list:
